1. Field of the Invention
This invention relates to hydrogen-absorbing alloy electrodes and alkaline storage batteries utilizing the hydrogen-absorbing alloy electrodes for their negative electrodes. More particularly, a feature of the invention is an improvement in a hydrogen-absorbing alloy electrode used in an alkaline storage battery, which utilizes, for the purpose of enhancing the capacity of the alkaline storage battery, a hydrogen-absorbing alloy containing at least a rare-earth element, magnesium, nickel, and aluminum and having an intensity ratio IA/IB of 0.1 or greater in X-ray diffraction analysis using Cu—Kα radiation as an X-ray source, where IA is the strongest peak intensity that appears in the range 2θ=31° to 33° and IB is the strongest peak intensity that appears in the range 2θ=40° to 44°, whereby the cycle life of the alkaline storage battery is improved.
2. Description of Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel-metal hydride storage batteries using a hydrogen-absorbing alloy as a material for their negative electrodes have drawn considerable attention from the viewpoints that they have higher capacity than nickel-cadmium storage batteries and, being free of cadmium, they are more environmentally safe.
As the nickel-metal hydride storage batteries have been used in various portable devices, demands for further higher performance in the nickel-metal hydride storage batteries have been increasing.
In the nickel-metal hydride storage batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni have been generally used for their negative electrodes.
However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to increase the capacity of the nickel-metal hydride storage batteries further.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a hydrogen-absorbing alloy having a Ce2Ni7 type or a CeNi3 type crystal structure, rather than the CaCu5 type, by adding Mg or the like to the rare earth-nickel hydrogen-absorbing alloy. (See, for example, Japanese Published Unexamined Patent Application No. 11-323469.)
Nevertheless, a problem in using the above-described hydrogen-absorbing alloy for the negative electrode of an alkaline storage battery has been that repeated charge-discharge cycles cause alkaline electrolyte solution within its separator to dry up, degrading the cycle life of the alkaline storage battery.